Poster at the 40th annual meeting of the Biopysical Society. Abstract: Biophys. J. 70, A196 Photolysis of photoactivatable or ~caged~ compounds provides a means of spatially and temporally controlling the release of biologically active molecules such as metabolites, neurotransmitters or calcium. The use of two photon excitation provides 3D spatial control and limits the region of potentially deleterious effects of UV light. The two photon excitation spectra and cross-sections were measured for carboxy-2-nitrobenzl ester caged GABA over the wavelength range 630 to 700nm using HPLC analysis of photoproduct, and for 4,5 - dimethoxy-2-nitrobenzl ester caged fluorescein over the wavelength range 680nm to 760nm using a calibrated fluorescence standard. In addition, several aspects related to the practical use of these caged compounds were measured, including pH dependence of photorelease, stability at 37C, percent initial uncaged effector (especially important to the use of caged neurotransmitters), and potential for membrane damage from focused illumination.